I prefer the formula VD = 1.73xLxIxR/1,000. The R is resistance per 1,000 feet from Table 9 of Chapter 9. L is the distance from the source to the load in feet. Note that the impedance varies with the pipe type. But is not significant except with higher ampacities. For your parameters using PVC conduit I get: VD = 1.73x475x70x0.31/1,000 = 17.83 volts. Pretty close.Use 2 instead of 1.73 for single phase.